[Dino] builds a simple non-contact voltage detector

[Dino] is back with another installment of his Hack a Week series, and in this episode he is taking on what he promises will be the last transistor-based project – at least for a little while.

In the video embedded below, he shows off a homemade voltage detector circuit that he constructed using a trio of BC547 NPN transistors. The circuit is pretty simple though very useful all the same. At one end, the device has a small copper strip, which is connected to the base of the first transistor. The emitter of that transistor is daisy chained to the base of the second transistor and so on, until reaching the indicator LED.

As noted by one of [Dino’s] viewers, the circuit functions as follows:

“The front end copper strip forms one side of a capacitor, and then when you bring it near a voltage potential a super tiny current flows between air dielectric of the “cap”. This is mega amplified with the high gain BC547′s and viola, the LED lights up.”

Since the small bit of current is amplified many times over, the LED lights up even when very small voltages are present. While we might not necessarily trust our lives to [Dino’s] voltage detector, we’re sure it would come in handy now and again.

So basically this is a simple field strength meter, that uses a LED indicator rather that a meter? One that use a high gain amplifier to make up for a less than ideal antenna at most frequencies, and a weak electromagnetic field. While not particularly useful, it would be a fun circuit to introduce to newcomers to electronics. The cat had a bit of attitude. Like if I wanted up here I’d wold jump up here when you don’t want be here

No matter how I determine if a circuit is dead, I still work on it if it’s a live circuit. I do my best to remember not to grab a conductor with out first bushing the buck of it with a finger. Giver the number of times that practice prevented from grabbing live metal electrical inclosures in the oil field I pretty much remember.

In one internet forum I ran across a fella that says to be sure the branch circuit is dead he shorts it out to trip the breaker. I replied asking if he was aware in of the amount a current a grid connected could deliver under short circuit condition, and hoe dangerous or, destructive his “safety” practice could be? In the event I was able to find the supporting evidence I seen in the ’70s in in school on the internet, he is unlikely to believe me,and he comes across on of those who wouldn’t believe it if I could.

Not that I sure or, more informed, I though simple touch detection circuits detected a change in circuit capacitance. Maybe that’s why Dino mentioned that the the copper strip acted as one side of a capacitor in the write up on his blog page? [shrug]

I remember building one of these by accident when I was like 10. Using chained NPN PNP NPN PNP – if the chain was long enough and the LED lit when not “near” a source, just my moving around near it would change the brightness. If hooked to a speaker, you can clearly hear the EMF (60Hz usually)

@teeh ah, somebody beat me to it! The ghost link: The circuit diagram is just three transistors, but the circuit you see him waving about appears to be more complicated. I don’t suppose you know where there’s a diagram for it? It would have been interesting to add a speaker to that one!

question?
1) Can I low sensitivity of the detector by change some resistance.
2) Can I measure in the water by cover the contact with shield ? I have done it work, but wanna test for safty area that we should not get close by. (flooding in Thailand)

I know this is an old post, but it would be great if you can give me an answer. My question is, how do you decide the values for the Resistors? Why did you choose the values 1 Mega ohm and 100 kilo ohm? How does it effect the circuit?